1. Field of the Invention
The present invention relates generally to systems for testing water chemistry, and, more particularly, relates to amperometric sensors.
2. Description of the Related Art
A need exists for a simple, reliable long life chlorine (or bromine) measurement system that can also measure both high levels of chlorine (or bromine) and low levels of chlorine (or bromine) with high accuracy over a wide range of levels. The system should also operate reliably in conditions that may cause scaling and that may result in biofouling of the sensor.
A common problem encountered with online measurement of chlorine or bromine in the field is fouled electrodes. Electrodes measurements can be rendered unreliable when the working electrode is covered with either inorganic (salts such as calcium carbonate) layers or organic (biofouling) layers that inhibit electrode processes.
Pulsing techniques are often used to clean the electrodes to provide repeatable measurements. Some examples of pulsing techniques are shown in U.S. Pat. No. 6,238,555 for Amperometric Halogen Control System and in U.S. Pat. No. 6,270,680 for Amperometric Sensor Probe for an Automatic Halogen Control System. While pulsed techniques are widely used, problems arise when using this method with certain electrode materials and when used in online control systems for water treatment. For example, gold works well as a working electrode in a chlorine measurement system. To clean the working electrode, a positive pulse greater than 1 volt must be applied to generate protons to clean salts from the surface. To merely achieve oxidation on the surface, a DC potential of 0.7 volts or more must be applied with respect to an Ag/AgCl electrode. This potential is very near the potential at which the gold electrode may be damaged from irreversible oxide formation.
Another disadvantage of pulsed techniques is the frequency of the measurement. Since several minutes of pulsing and stabilization are typically required, many minutes may elapse between measurements. While not an issue in slowly responding systems such as a swimming pool, this time may be unacceptably long for municipal water systems or in a hot tub or commercial spa.
Censar Technologies, Inc., (a unit of Siemens AG) uses a replaceable thin-film sensor formed on a substrate with multiple electrodes. The sensor has a very short life of approximately 6 months. The embodiments in accordance with aspects of the invention disclosed herein use novel circuitry to use the chlorine measurement electrodes for multiple measurements. The use of the novel circuitry results in a robust, reliable sensor that is lower in cost than the Censar approach. See, for example, U.S. Pat. No. 5,483,164 to Moss et al.
Another system uses an impeller to move cleaning balls that abrade the surface of the electrode.
Other systems use a fixed potential to measure chlorine. These systems usually take 24 hours or more to stabilize and are subject to fouling and frequent calibration.
Membrane sensors operate reliably in drinking water applications but often foul under conditions that include high levels of organics or other contaminants. Membrane sensors require frequent maintenance and recalibration and cannot be used in high pressure applications.
Oxidation reduction potential (ORP) is a method that is commonly used in swimming pools as a substitute for chlorine control. This method has a number of deficiencies including non-specificity. The ORP method measures the sum total of all redox couples in the water, not just chlorine. The method exhibits a logarithmic response to the chlorine level and is easily poisoned by organics, including cyanuric acid, a chemical that can easily reach excessive levels in swimming pools. Reports of levels as high as 350 ppm are common. When such high levels are reached, ORP sensors reportedly must be cleaned every three days.
While the ORP method is not always the best choice for a chlorine controller, the method can be used as a qualitative indicator of water quality and works well in some process applications.
Keeping amperometric sensors clean (e.g., avoiding scale build up) is an issue with water chemistry measurement systems. Ultrasonic energy is widely used for cleaning applications. Wissenschaftlich-Technische Werkstaetten GmBH (WTW) of Weilheim, Germany, sells an ultrasonically cleaned optical sensor for wastewater suspended solids measurement, such as, for example, the sensor disclosed in U.S. Pat. No. 6,678,045. An ultrasound generating transducer is built into the sensor tip. The transducer is electronically activated to produce ultrasonic waves in the electrodes. In ultrasonic cleaning, the main mechanism of cleaning action is by energy released from the creation and collapse of microscopic cavitation bubbles, which break up and lift off dirt and contaminants from the surface to be cleaned.
Ultrasonic transducers work by rapidly changing size when excited by an electrical signal. This creates a compression wave in the liquid. The compression waves actually “tear” the liquid apart, leaving behind a “void” or “partial vacuum bubble.” When these “bubbles” (e.g., millions of bubbles) collapse, enormous energy is produced. When sufficient energy is built up in the “bubble” or cavitation, the cavitation collapses violently.
Another known system available from Emerson Process Management utilizes a separate chamber into which a conventional pH sensor can be inserted. Water flows through the chamber and ultrasonic energy is applied the chamber wall. This device is extremely expensive and uses very high voltages (greater than 500 volts). Since the energy is applied to the water instead of directly applying it to the electrodes, the device is both cumbersome and inefficient.